Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system may allow the vapors to be purged into an engine intake manifold for use as fuel.
The purging of fuel vapors from the fuel vapor canister may involve opening a canister purge valve coupled to a conduit between the fuel vapor canister and the intake manifold. During a purge operation, vacuum or negative pressure in the intake manifold may draw air through the fuel vapor canister enabling desorption of fuel vapors from the canister. These desorbed fuel vapors may flow through the canister purge valve into the intake manifold. As such, the canister purge valve may regulate the flow of fuel vapors into the intake manifold via a sonic choke positioned in series with the canister purge valve. Accordingly, the sonic choke may function as a flow restrictor in the purge path between the valve and the intake manifold.
In boosted engines, during boost conditions when the compressor is operative, the intake manifold may have a positive pressure. Herein, an ejector coupled in a compressor bypass passage may generate vacuum that can be used to draw stored fuel vapors from the fuel vapor canister. However, purge flow through the ejector may be lower because the sonic choke in the canister purge valve may excessively restrict canister purge flow to the suction port of the ejector. Accordingly, a performance of the ejector in terms of purging the fuel vapor canister may be severely diminished by the presence of the sonic choke in the flow path.
An example approach demonstrating an improved purging operation is shown by Stephani in DE 011084539. Herein, an ejector coupled in the compressor bypass passage directly communicates with the fuel vapor canister such that fuel vapors are purged to the ejector from the fuel vapor canister without flowing through a canister purge valve. By directly coupling the fuel vapor canister to the ejector, the metering effect of the sonic choke in the canister purge valve may be circumvented. A diverter valve in the compressor bypass passage regulates flow through the ejector and therefore, purging of the fuel vapor canister.
The inventors herein have identified potential issues with the above approach. The approach in DE 102011084539 is primarily used during non-idle conditions when the ejector can generate a vacuum to draw purged fuel vapors. Thus, boost levels must be high enough to generate a sufficient vacuum at the ejector for drawing purged fuel vapors. As such, during lower boost levels, the purging efficiency may be reduced. Accordingly, manifold vacuum during idle conditions may not be availed for canister purging.
The inventors herein have recognized the above issues and identified an approach to at least partly address the issues. In one example approach, a method may comprise: during boosted conditions, generating vacuum by recirculating compressed air through an ejector coupled in a compressor bypass passage, applying a first portion of the vacuum on a purge line downstream of a canister purge valve, and applying a second, remaining portion of the vacuum on the purge line upstream of the canister purge valve.
The method may additionally or alternatively comprise adjusting a ratio of the first portion of vacuum relative to the second portion of vacuum applied based on one or more of a canister load, a time since a previous purge, intake manifold vacuum level, and boost level. Adjusting the ratio may in some examples include, as the canister load increases, increasing the first portion of the vacuum applied on the purge line downstream of the canister purge valve relative to the second portion of the vacuum applied on the purge line upstream of the canister purge valve.
In another representation, a method for a boosted engine may comprise: during a first condition, flowing stored fuel vapors from a canister into an intake manifold via a canister purge valve, during a second condition, flowing stored fuel vapors from the canister into a suction port of an ejector coupled in a compressor bypass passage, the stored fuel vapors flowing through a bypass passage circumventing the canister purge valve, and during a third condition, flowing stored fuel vapors from the canister into each of the suction port of the ejector and the intake manifold via the canister purge valve, the stored fuel vapors flowing into the suction port of the ejector via each of the canister purge valve and a check valve.
In some examples, during the first condition, the stored fuel vapors may not flow through a bypass valve coupled in the bypass passage or the check valve, during the second condition, the stored fuel vapors may not flow through the purge valve or the check valve, and during the third condition, the stored fuel vapors do not flow through the bypass valve. Additionally, the first condition may include engine operation with natural aspiration, and each of the second and third conditions may include engine operation with boost, a boost level during the second condition being higher than the boost level during the third condition.
In this way, the amount of fuel vapors that may be purged from a fuel vapor canister during boosted conditions in a turbocharged engine may be increased. Further, the amount of fuel vapors that may be purged from a fuel vapor canister during shallow intake manifold vacuum levels may be increased. By coupling the ejector to the fuel vapor canister via two separate flow paths, one through a canister purge valve, and the other through a bypass valve, the canister purge valve may be circumvented and a purge flow rate to the compressor inlet may be enhanced during boosted conditions where manifold vacuum is lower. Further, by controlling compressor bypass flow and ejector vacuum via an ejector shut-off valve based on engine conditions, engine performance may be enhanced. Overall, vehicle fuel economy and emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.